A conventional optical fiber laser featuring a dual clad fiber (DCF) amplifier can include a laser diode based pump source for emitting laser pump light, an optical fiber having a core doped with rare-earth ions, an inner cladding, and an outer cladding surrounded by an outer protective jacket. The pump source can be, for example, high brightness arrays of laser diode bars based on GaAlAs, GaInPAs, GaInAlAs, GaInAs or similar active layers. The rare-earth ions can be, for example, ytterbium (Yb), ytterbium/erbium (Yb/Er), or thulium (Tm), each lasing at different wavelengths in combination with appropriate diode pump wavelengths.
A conventional Tm-doped fiber laser pumped with standard diodes for emitting 790 nm light can emit a signal wavelength at 2000 nm. Thus, the conventional Tm-doped fiber laser has a maximum optical-to-optical efficiency of approximately 40% (790/2000). Under certain conditions, the optical-to-optical efficiency of a conventional Tm-doped fiber laser can theoretically be doubled by the so-called cross-relaxation process in which the 3F4-3H6 pump transitions of Tm3+ near 800 nm are utilized as shown in FIG. 4. The Tm3+ cross-relaxation is a nonradiative process in which a single excited Tm3+ ion in the 3H4 level generates two Tm3+ ions in the 3F4 upper laser level, theoretically doubling the maximum optical-to-optical efficiency from 40% to 80%. The Quantum Efficiency (the number of photons emitted divided by the number of pump photons absorbed) is thus effectively doubled.
The mean inter-ionic distance must be sufficiently small such that the wave functions for the two ions can interact to achieve the cross-relaxation. Thus, the doping concentration of Tm3+ must be sufficiently high. However, the high concentration of Tm3+ leads to the drawback of increased density of heat generation, thereby resulting in high heat loads per unit length from intense pumping, and prohibiting high power applications through excessive temperature rise. The high heat loads can also cause fiber fusion (the fiber fuse phenomenon), in which the core of the fiber is destroyed by a moving melt zone driven by laser power absorption.
One approach to dealing with the high heat loads has been to provide robust fiber cooling means such as liquid nitrogen flowing around the fiber. However, such cooling means can not remove heat fast enough to prevent the initiation of fiber fusion at fiber power levels of interest.
Another approach to dealing with the high heat loads has been to reduce the size of the fiber core. However, this approach has the drawback of limited fiber power as a result of the intense beams inducing Brillouin scattering from acoustic vibrations, a phenomenon known as Stimulated Brillouin Scattering (SBS). Hence, restricting the core size to manage the heat load leads to limitations in the useful fiber power.
It would be desirable to have an optical fiber laser that can utilize the cross-relaxation process for operational use while having a sufficiently low heat load, i.e., power dissipated per unit fiber length. It would be further desirable for such an optical fiber laser to generate light with a wavelength within the eye safe spectral region for which the wavelength (λ) is greater than approximately 1.5 microns.